Soil permeameter is a device in the field of hydraulics, used to measure the permeability property of soil/rock. Permeability is an index of interconnectivity of pores. The coefficient of permeability is a constant of proportionality relating to the ease with which fluid passes through a porous medium. This parameter is very critical in understanding the fluid flow process in porous media and has a wide application in hydraulic engineering and fluid transport modeling studies.
Soil hydraulic conductivity has been historically measured in the laboratory, utilizing a falling or constant head of water applied to soil core samples retrieved from the field or on remolded soil samples. Laboratory measurements are often significantly at variance with in-situ field measurements because of the differing methodologies and the inherent difficulty of obtaining undisturbed soil samples. The hydraulic conductivity of soils at different depths is highly variable due to heterogeneous textural arrangement of soil particles.
It is desirable to have the capability to conduct hydraulic conductivity tests in laboratory by having the undisturbed soil in the form of a core of any desired depth above the permanent water table. Such depths may range from zero to many meters below the ground surface. In addition, it is desirable to have adequate flow capacity for maintaining flow equilibrium in a wide range of soils. Clay soils often have low permeability, whereas sandy or gravelly soils often have high permeability and, therefore, a greater accuracy is necessary in the measurement of time in case of falling head permeameter where the time reflects the permeability characteristics of soil under testing.
Prior art instruments developed for measuring hydraulic conductivity of soils generally fall into two major categories, namely- the lab measurements and in-situ field measurements. In the first type, the soil is collected from the field and subjected to permeability measurement in the lab. The second type is of measuring the permeability of soil at in-situ condition. For the first category of lab measurements, two types of permeameters are available, out of which one applies a constant head and the other a falling head. Both these types apply in principle Darcy's Law for calculation of coefficient of permeability. The second category applied for in-situ measurement of permeability utilizes various methodologies, which include electrical resistivity procedures and gas or liquid injection into the soil through penetrating probes and measuring permeability of unsaturated & saturated regime and complex analysis procedures.
The laboratory measurement of permeability is simpler, but requires collection of soil from the site, safe transportation to the lab, careful setting of lab experiment, and accuracy of measurements and reproducibility of experimental results. Among these two methods of measuring the permeability, namely the constant head has been reported, to be suitable for measuring the permeability of higher ranges, i.e, for coarser soil of more than 200 microns, while the falling head permeameter is for soils less than 200 microns having lower permeability.
The continuing physico-chemical processes ultimately disintegrate the rock into a fine soil texture and deposit in a suitable environment. In most of the semi arid environmental conditions, witnessing regular monsoon cycle, quick removal of the disintegrated rock materials and transport them to places of farther away from place of origin making them further finer particles and gets deposited as low permeable soil layers.
Reference may be made to U.S. Pat. No. 4,072,044 (Farwell et al 1976), U.S. Pat. No. 4,099,406 (Fulkerson, et al 1977) and U.S. Pat. No. 4,969,111 (Merva et al 1990) and scientific literature cited, indicating that the falling head permeameter is preferable for low permeability ranges and several errors/constraints that could affect the test results as reported are:
air trapped in sample; accuracy on measuring the elapsed time of test; uniform supply of water at the head soil core sample; disturbed soil conditions while loading the sample in apparatus; measurement error in head at beginning and at the end of test and area of specimen.